For machines that are not directly driven by their respective power sources, a transmission is a critical component of the drive train, affecting both performance and efficiency. Transmissions fulfill many roles, including, for example, gear reduction or amplification to match final drive speed and/or torque to engine speed and/or torque, connection and disconnection between the power source and the final drive, drive train shock absorption, machine energy absorption, i.e., during machine slowing, and so on. While the fulfillment of many of these goals requires a certain amount of complexity within the transmission system, this same complexity can lead to problems in transmission controllability and stability.
Hydraulic transmissions and drives can be used to great benefit in many scenarios, but are fairly complex. Such transmissions include without limitation hystat, hydromechanical, or other transmissions or drives that include a hydraulic variator or hydraulic pump/motor system. One of the more useful but complex hydraulic transmission systems is the hydromechanical split torque (or parallel path) transmission, which will be discussed by way of example herein. This transmission type provides numerous advantages over typical mechanical transmissions used in earth-working machines, such as tractors, bulldozers, and wheel loaders. For example, a hydromechanical transmission is typically able to provide continuous speed control and more effective and efficient management of engine speed.
Due to the complexity of such transmissions however, their mode of control is quite different from the techniques by which standard mechanical transmissions are controlled. As will be discussed in detail below, a hydromechanical split torque transmission includes a variator whose output is tied to the primary power source, usually an engine, via a set of planetary gear systems within the transmission. The variator includes a hydrostatic pump that has a displacement that is influenced by an actuator.
The transmission output is thus a function of the instantaneous characteristics of both the engine and the variator. Traditionally, such hydromechanical transmissions are controlled by speed control techniques. In particular, the transmission output speed is controlled by controlling pump displacement. This practice uses feedback control on the pump actuator to force the actuator to a specific position, thus enforcing a specific pump displacement. While such speed control systems provide torque at the output, they do not control the output torque, just the output speed or speed ratio (an output speed over an input speed). This system has been effective in certain industries; however, the lack of torque control is disadvantageous in a number of industries and environments, especially those involving the use of large earth-working machines.
For example, when a speed-controlled machine encounters a sudden resistance due to load or grade, the speed-controlled transmission may cause the engine to lug as it maintains speed ratio or speed, or may cause other instabilities or undesirable behavior with respect to the drive train. One type of system for controlling speed is discussed in U.S. Pat. No. 6,684,636 to Smith. Smith teaches a method for controlling speed using an electrical signal applied to a solenoid to change the pump's displacement. On generally level surfaces, this method has been successful, however, as discussed above, on uneven surfaces, an operator, or machine experiences undesirable accelerations as the controls hunt for the desired speed.
Thus, while speed-controlled transmissions could in theory be expected to meet the needs of earth-working industries, the system has not been widely successful in practical application, due to the difficulty in determining correct pump displacement under rapidly changing surface conditions. In particular, any error in the displacement during speed control can cause lugs and lurches and generally undesirable machine behavior.
The disclosed principles herein are directed at least in part to overcoming one or more disadvantages of the prior art, noted or otherwise. However, it will be appreciated that the invention itself is defined by the attached claims without regard to whether and to what extent the specifically claimed invention overcomes one or more of the noted problems in the existing technology. Moreover, it will be appreciated that any discussion herein of any reference or publication is merely intended as an invitation to study the indicated reference itself, and is not intended to replace or supplement the actual reference. To the extent that the discussion of any reference herein is inconsistent with that reference, it will be appreciated that the reference itself is conclusive as to its teachings.